THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN FROM A DISTINCT PLANT.

We have seen in four cases that seedlings raised from a cross between flowers on the same plant, even on plants appearing distinct from having been propagated by stolons or cuttings, were not superior to seedlings from self-fertilised flowers; and in a fifth case (Digitalis) superior only in a slight degree. Therefore we might expect that with plants growing in a state of nature a cross between the flowers on distinct individuals, and not merely between the flowers on the same plant, would generally or often be effected by some means. The fact of bees and of some Diptera visiting the flowers of the same species as long as they can, instead of promiscuously visiting various species, favours the intercrossing of distinct plants. On the other hand, insects usually search a large number of flowers on the same plant before they fly to another, and this is opposed to cross-fertilisation. The extraordinary number of flowers which bees are able to search within a very short space of time, as will be shown in a future chapter, increases the chance of cross-fertilisation; as does the fact that they are not able to perceive without entering a flower whether other bees have exhausted the nectar. For instance, Hermann Muller found that four-fifths of the flowers of Lamium album which a humble-bee visited had been already exhausted of their nectar. (10/29. ‘Die Befruchtung’ etc. page 311.) In order that distinct plants should be intercrossed, it is of course indispensable that two or more individuals should grow near one another; and this is generally the case. Thus A. de Candolle remarks that in ascending a mountain the individuals of the same species do not commonly disappear near its upper limit quite gradually, but rather abruptly. This fact can hardly be explained by the nature of the conditions, as these graduate away in an insensible manner, and it probably depends in large part on vigorous seedlings being produced only as high up the mountain as many individuals can subsist together.

With respect to dioecious plants, distinct individuals must always fertilise each other. With monoecious plants, as pollen has to be carried from flower to flower, there will always be a good chance of its being carried from plant to plant. Delpino has also observed the curious fact that certain individuals of the monoecious walnut (Juglans regia) are proterandrous, and others proterogynous, and these will reciprocally fertilise each other. (10/30. ‘Ult. Osservazioni’ etc. part 2 fasc 2 page 337.) So it is with the common nut (Corylus avellana) (10/31. ‘Nature’ 1875 page 26.), and, what is more surprising, with some few hermaphrodite plants, as observed by Hermann Muller. (10/32. ‘Die Befruchtung’ etc. pages 285, 339.) These latter plants cannot fail to act on each other like dimorphic or trimorphic species, in which the union of two individuals is necessary for full and normal fertility. With ordinary hermaphrodite species, the expansion of only a few flowers at the same time is one of the simplest means for favouring the intercrossing of distinct individuals; but this would render the plants less conspicuous to insects, unless the flowers were of large size, as in the case of several bulbous plants. Kerner thinks that it is for this object that the Australian Villarsia parnassifolia produces daily only a single flower. (10/33. ‘Die Schutzmittel’ etc page 23.) Mr. Cheeseman also remarks, that as certain Orchids in New Zealand which require insect-aid for their fertilisation bear only a single flower, distinct plants cannot fail to intercross. (10/34. ‘Transactions of the New Zealand Institute’ volume 5 1873 page 356.)

Dichogamy, which prevails so extensively throughout the vegetable kingdom, much increases the chance of distinct individuals intercrossing. With proterandrous species, which are far more ccommon than proterogynous, the young flowers are exclusively male in function, and the older ones exclusively female; and as bees habitually alight low down on the spikes of flowers in order to crawl upwards, they get dusted with pollen from the uppermost flowers, which they carry to the stigmas of the lower and older flowers on the next spike which they visit. The degree to which distinct plants will thus be intercrossed depends on the number of spikes in full flower at the same time on the same plant. With proterogynous flowers and with depending racemes, the manner in which insects visit the flowers ought to be reversed in order that distinct plants should be intercrossed. But this whole subject requires further investigation, as the great importance of crosses between distinct individuals, instead of merely between distinct flowers, has hitherto been hardly recognised.

In some few cases the special movements of certain organs almost ensure pollen being carried from plant to plant. Thus with many orchids, the pollen-masses after becoming attached to the head or proboscis of an insect do not move into the proper position for striking the stigma, until ample time has elapsed for the insect to fly to another plant. With Spiranthes autumnalis, the pollen-masses cannot be applied to the stigma until the labellum and rostellum have moved apart, and this movement is very slow. (10/35. ‘The Various Contrivances by which British and Foreign Orchids are fertilised’ first edition page 128.) With Posoqueria fragrans (one of the Rubiaceae) the same end is gained by the movement of a specially constructed stamen, as described by Fritz Muller.

We now come to a far more general and therefore more important means by which the mutual fertilisation of distinct plants is effected, namely, the fertilising power of pollen from another variety or individual being greater than that of a plant’s own pollen. The simplest and best known case of prepotent action in pollen, though it does not bear directly on our present subject, is that of a plant’s own pollen over that from a distinct species. If pollen from a distinct species be placed on the stigma of a castrated flower, and then after the interval of several hours, pollen from the same species be placed on the stigma, the effects of the former are wholly obliterated, excepting in some rare cases. If two varieties are treated in the same manner, the result is analogous, though of directly opposite nature; for pollen from any other variety is often or generally prepotent over that from the same flower. I will give some instances: the pollen of Mimulus luteus regularly falls on the stigma of its own flower, for the plant is highly fertile when insects are excluded. Now several flowers on a remarkably constant whitish variety were fertilised without being castrated with pollen from a yellowish variety; and of the twenty-eight seedlings thus raised, every one bore yellowish flowers, so that the pollen of the yellow variety completely overwhelmed that of the mother-plant. Again, Iberis umbellata is spontaneously self-fertile, and I saw an abundance of pollen from their own flowers on the stigmas; nevertheless, of thirty seedlings raised from non-castrated flowers of a crimson variety crossed with pollen from a pink variety, twenty-four bore pink flowers, like those of the male or pollen-bearing parent.

In these two cases flowers were fertilised with pollen from a distinct variety, and this was shown to be prepotent by the character of the offspring. Nearly similar results often follow when two or more self-fertile varieties are allowed to grow near one another and are visited by insects. The common cabbage produces a large number of flowers on the same stalk, and when insects are excluded these set many capsules, moderately rich in seeds. I planted a white Kohl-rabi, a purple Kohl-rabi, a Portsmouth broccoli, a Brussels sprout, and a Sugar-loaf cabbage near together and left them uncovered. Seeds collected from each kind were sown in separate beds; and the majority of the seedlings in all five beds were mongrelised in the most complicated manner, some taking more after one variety, and some after another. The effects of the Kohl-rabi were particularly plain in the enlarged stems of many of the seedlings. Altogether 233 plants were raised, of which 155 were mongrelised in the plainest manner, and of the remaining 78 not half were absolutely pure. I repeated the experiment by planting near together two varieties of cabbage with purple-green and white-green lacinated leaves; and of the 325 seedlings raised from the purple-green variety, 165 had white-green and 160 purple-green leaves. Of the 466 seedlings raised from the white-green variety, 220 had purple-green and 246 white-green leaves. These cases show how largely pollen from a neighbouring variety of the cabbage effaces the action of the plant’s own pollen. We should bear in mind that pollen must be carried by the bees from flower to flower on the same large branching stem much more abundantly than from plant to plant; and in the case of plants the flowers of which are in some degree dichogamous, those on the same stem would be of different ages, and would thus be as ready for mutual fertilisation as the flowers on distinct plants, were it not for the prepotency of pollen from another variety. (10/36. A writer in the ‘Gardeners’ Chronicle’ 1855 page 730, says that he planted a bed of turnips (Brassica rapa) and of rape (B. napus) close together, and sowed the seeds of the former. The result was that scarcely one seedling was true to its kind, and several closely resembled rape.)

Several varieties of the radish (Raphanus sativus), which is moderately self-fertile when insects are excluded, were in flower at the same time in my garden. Seed was collected from one of them, and out of twenty-two seedlings thus raised only twelve were true to their kind. (10/37. Duhamel as quoted by Godron ‘De l’Espece’ tome 2 page 50, makes an analogous statement with respect to this plant.)

The onion produces a large number of flowers, all crowded together into a large globular head, each flower having six stamens; so that the stigmas receive plenty of pollen from their own and the adjoining anthers. Consequently the plant is fairly self-fertile when protected from insects. A blood-red, silver, globe and Spanish onion were planted near together; and seedlings were raised from each kind in four separate beds. In all the beds mongrels of various kinds were numerous, except amongst the ten seedlings from the blood-red onion, which included only two. Altogether forty-six seedlings were raised, of which thirty-one had been plainly crossed.

A similar result is known to follow with the varieties of many other plants, if allowed to flower near together: I refer here only to species which are capable of fertilising themselves, for if this be not the case, they would of course be liable to be crossed by any other variety growing near. Horticulturists do not commonly distinguish between the effects of variability and intercrossing; but I have collected evidence on the natural crossing of varieties of the tulip, hyacinth, anemone, ranunculus, strawberry, Leptosiphon androsaceus, orange, rhododendron and rhubarb, all of which plants I believe to be self-fertile. (10/38. With respect to tulips and some other flowers, see Godron ‘De l’Espece’ tome 1 page 252. For anemones ‘Gardeners’ Chronicle’ 1859 page 98. For strawberries see Herbert in ‘Transactions of the Horticultural Society’ volume 4 page 17. The same observer elsewhere speaks of the spontaneous crossing of rhododendrons. Gallesio makes the same statement with respect to oranges. I have myself known extensive crossing to occur with the common rhubarb. For Leptosiphon, Verlot ‘Des Varieties’ 1865 page 20. I have not included in my list the Carnation, Nemophila, or Antirrhinum, the varieties of which are known to cross freely, because these plants are not always self-fertile. I know nothing about the self-fertility of Trollius Lecoq ‘De la Fecondation’ 1862 page 93, Mahonia, and Crinum, in which genera the species intercross largely. With respect to Mahonia it is now scarcely possible to procure in this country pure specimens of M. aquifolium or repens; and the various species of Crinum sent by Herbert ‘Amaryllidaceae’ page 32, to Calcutta, crossed there so freely that pure seed could not be saved.) Much other indirect evidence could be given with respect to the extent to which varieties of the same species spontaneously intercross.